[CM] make-pulsed-env

[bil at ccrma.Stanford.EDU]

The make-pulsed-env frequency parameter sets the
base frequency (it can be 0); you change the pulse
rate while running via the fm argument to the
pulsed-env generator.  For example:

(with-sound ()
   (let ((e (make-pulsed-env '(0 0 1 1 2 0) .01 1))
         (frq (make-env '(0 0 1 1) :duration 1.0 :scaler (hz->radians 
50))))
     (do ((i 0 (+ i 1)))
         ((= i 44100))
       (outa i (* .5 (pulsed-env e (env frq)))))))